Polysaccharide : introduction, Classification, properties

Polysaccharide : introduction, Classification, properties

Introduction of Polysaccharides
These are carbohydrates which can be hydrolysed to many n = (100-3000) of monosaccharide units. They are classified as homo-polysaccharides or hetero-polysaccharides, depending upon whether they give only one kind or more than one kind of monosaccharide units on hydrolysis. Their general formula is (C6H10O5)n

Characteristics of Polysaccharides

1. Polysaccharides are insoluble in water or form a colloidal solution.

2. They are colourless.

3. They are tasteless or they are not swept as sugars are.

4. Polysaccharides are macromolecules, formed by thousands of monosaccharide molecules. They are represented by general formula (C6H10O5)n

5. On hydrolysis they produce mono-saccharides molecules.


Classification of polysaccharides 

Depending upon the composition, these are of two types-

[A] Homopolysaccharides 

Polysaccharides that formed by the condensation of the same type of monosaccharides are known as homopolysaccharides. The chief homopolysaccharides are cellulose, starch, glycogen and dextrin. These are formed by condensation of glucose.

1. Cellulose - Cellulose molecule is composed of 1600 to 2700 B-D-glucose units with 1-4 glycosidic linkage. It is a long thread like molecule. Haworth Formula-

Cellulose is the major structural polysaccharide of plants. Careful Acid hydrolysis of cellulose yields a range of oligosaccharides such as cellobiose, cellotriose, cellotetraose, etc. Complete hydrolysis permits recovery of about 95% of the weight of cellulose as glucose. Enzymatic hydrolysis of cellulose is brought about by cellulase which is present in some protozoans and mites. Some of the bacteria in cattle and other herbivores also secrete cellulase that enables the cattles to digest the herbal cellulose to glucose.

2. Starch - It is most important storage polysaccharide in the plant kingdom and found in abundance in rice, wheat, cereals, legumes, potato & banana etc. It contains amylose and amylopectin, two different kinds of glucose polymers.

• Amylose - Amylose consists of long, unbranched chains of D-glucose residues connected by a-1-4 linkage. It constitutes about 20% of starch.
• Amylopectin - Amylopectin is branched chain of glucose residues. It constitutes about 80% of starch.

3. Glycogen - It is the main storage polysaccharide of animal cells. It is mainly stored in the muscles and liver of mammals. it is long and much branched chain of a-D glucose units. The glucose units are linked by a-1, 4 glycosidic linkage but at branching point these units are connected- through a 1-6 linkage.

4 Inulin - Inulin is an unbranched molecule of approximately 30–35 D-fructose units, m.w. 5000 and the glycosidic linkage is between carbon 1 and 2 of the fructose units. Inulin is obtained commercially from Jerusalem artichoke rhizome. It is found naturally with other short chain fructan. Enzyme enolase hydrolysis inulin to fructose.


[B] Hetero Polysaccharide

Polysaccharides that formed by condensation of more than one type of monosaccharides are known as hetero Polysaccharides. These participates in the formation of tissues. They also condense with proteins hence, also known as mucopolysaccharides.

(i) Heparin - It is formed by condensation of glucosamine, gluconic acid and sulphuric acid. It is secreted by mast cell in the intestinal mucosa, liver, lung, spleen and kidney. It is present in circulating medium and acts as anticoagulant i.e. prevents clotting of blood.

(ii) Chondriotin sulphate - This is formed by condensation of D- glucuronic acid, N-acetyl galactosamine and sulphuric acid. It is present in cartilage, tendon, skin and connective tissue of organisms.

(iii) Hyaluronic acid - This is formed of equimolar condensation of N-acetylglucosamine and D- glucuronic acid. It is found in the connective tissue and body mucous fluids.

(iv) Blood group substances - These are formed by condensation of N-acetylglucosamine, galactosamine galactose,. fructose and sialic acid. These determine blood groups.

(v) Glycoproteins (mucoproteins) - These are protein polysaccharide compounds occuring in the tissues particularly in mucous secretion. Example Ovomucoid, Ovalbumin Submaxillary mucins, Fibrinogen, T-globulins of serum, Urinary glycopeptides.

(vi) Chitin - It is structural polysaccharide found in exoskeleton of insects, crabs and lobsters and cell wall of fungi. It is similar to cellulose except that its basic unit is a derivative of glucose called N-acetyl glucosamine.


Biological significance of carbohydrates

1. Structural components of cells - In some animals and all plants, carbohydrates play an essential structural role. They construct the cellulose framework here.

2. Major source of energy - Carbohydrates are essential for life in all living organisms. Almost all animals use it as a respiratory fuel. In animal cells, carbohydrates in the form of glucose and glycogen play an important role. It serves as an important source of energy for these activities.

3. Key role in metabolism - In the metabolism of amino acids and fatty acids, carbohydrates are important.

4. Special functions - Some carbohydrates perform functions that are highly specialized, for example ribose in the nucleoproteins of cells, galactose in some lipids, and lactose in milk.